After components are soldered to a circuit board, the board undergoes a cleaning operation to remove any residues that remain following soldering. Circuit boards are usually cleaned by washing them with a solvent capable of dissolving the undesirable residues. While solvent washing is an effective cleaning method, disposing of the spent solvent in an environmentally correct manner often imposes severe constraints. For that reason, cleaning a circuit board with solid carbon dioxide (CO.sub.2) particles has become very attractive.
To clean a circuit board with solid CO.sub.2 particles, the particles, either in the form of pellets or snow, are directed at the surface of the board. Upon contact, the CO.sub.2 particles sublimate, effectively removing the residues on the board. Thus, the board surface is generally left clean and residue-free. The waste stream resulting from CO.sub.2 cleaning consists essentially of the residues removed from the board, imposing less environmental constraints than conventional solvent cleaning.
Despite the advantage of reduced environmental impact, CO.sub.2 cleaning is not without its drawbacks. In particular, CO.sub.2 cleaning usually results in electrostatic discharge (ESD), which is a serious problem. When solid CO.sub.2 particles, which are cold and dry, strike the board surface, the particles tribocharge the circuit board. The level of charge on the circuit board can become high enough to breakdown the air surrounding the board, thereby creating ESD events. Certain electronic components are very sensitive to ESD events, and therefore may not be cleaned by CO.sub.2.
There are two possible approaches to reducing the ESD associated with CO.sub.2 cleaning. One possible approach is to reduce or eliminate tribocharging of the board. In practice, reducing tribocharging has not proved feasible. The other approach is to mitigate ESD during CO.sub.2 cleaning. One conventional method of mitigating ESD is to dissipate the charge generated during tribocharging by grounding the conductive areas on the circuit board. Unfortunately, charge may also build up on the non-conductive surfaces of the circuit board during CO.sub.2 cleaning. Such surfaces, by their very nature, cannot be grounded. Thus, grounding, by itself, will not reduce ESD below a level at which a circuit board containing ESD-sensitive components can be safely cleaned. Additionally, grounding the conductive areas on each circuit board is not practical for most in-line cleaning processes.
Another approach to mitigating ESD generated during CO.sub.2 cleaning is to spray the circuit board with a water mist to coat the surface with a film of water. The water film acts to dissipate charges built up during CO.sub.2 cleaning.
U.S. Pat. No. 5,651,834 to Lucent Technologies suggests spraying a water mist at the cleaning surface and at the CO.sub.2 snow stream. However, the water tends to freeze on contact with the cold CO.sub.2 stream thereby reducing its effectiveness in conducting the charge, as well as impeding the cleaning action of the CO.sub.2 stream. Another drawback of this technique is that an electrostatic charge is generated on the water particles as they are being atomized in spray nozzles, which counteracts the electrostatic charge dissipation.
Thus, there is a need for a technique for cleaning a circuit board with CO.sub.2 that mitigates ESD to sufficiently low levels acceptable for ESD-sensitive components.